This invention relates to bladders for game balls, and, more particularly, to a bladder which is formed from a single layer of film which is comprised of polyurethane and polyvinylchloride.
Sport balls, such as American-style footballs, soccerballs, and volleyballs, are constructed with an inner bladder that is inflated with air. The inflation of the bladder gives the sport ball its shape. When impacted, the bladder's resiliency allows the energy that is introduced to be returned. This return in energy propels the ball away from the point of impact.
For a sport ball 200 years ago, pig intestine was a suitable bladder material due to its elastic nature. With the introduction of rubber materials about 150 years ago, this material proved to be far superior material than an animal's intestine because of its increased durability and air retention characteristics.
Until recently, butyl rubber was the industry standard for the construction of sport ball bladders. Of the many synthetic and natural rubber compounds available, butyl rubber was the best choice because of its low gas transmission and good tensile strength and elasticity. Butyl rubber has been used for many years as the material in inner tubes for auto and bicycle tires.
Drawbacks to butyl rubber bladders include their expense due to its labor-intensive operation and capital-intensive requirements. Production of butyl rubber bladders is a dirty operation. Due to the porosity of the material, multiple layers are usually incorporated to safe guard against pin-hole leaks. Also, because of the softness of the material, butyl bladders result in high defective rates (15-20%) from leaks caused by a stray lacing awl or sewing needle which are used to shut the opening on a sport ball.
With the introduction of new thermoplastic elastomer films--specificially polyurethane--in the last 15 years, superior thermoplastic elastomer films can be purchased from film converters in a cured state. The film may be easily constructed into a bladder by heat- or radio-frequency sealing two or more layers and die-cutting the sealed layers to the desired shape.
Other thermoplastic elastomer materials for use in bladders include low-density polyethylene, polyvinylchloride, ethylene-vinyl-acetate, copolyester, polyamide, and polyethylene terapthalate. Of the thermoplastic films mentioned, thermoplastic, polyurethane (TPU) proved the best because of its excellent tensile strength, abrasion resistance, stretchability, durability and air retention properties. TPU materials are generally classified by the polyol used in its formulation. The two most common TPU films are polyester-type and polyether-type. The polyester-type TPU film has proven to be the new industry standard for bladders over the polyether-type TPU film because of its lower cost, superior tensile properties and durability, and lower gas transmission rate.
Since the early 1980's, polyester-type TPU film has been used in the production of bladders for sport balls. The problem is that polyester-type TPU materials are susceptible to hydrolysis which results in the air leaking through a hole in the bladder. Hydrolysis is the condition where water and stress result in the breakdown along the carbon chain found in the polymer. This hydrolysis phenomena is caused by the carbon atoms separating within the polyurethane molecule which results in microscopic tears (less than 1/64 inch) in the film and a passage for the pressurized air in the bladder to leak out.
Polyether-type TPU film exhibit the same tensile strength and durability as polyester-type TPU films. However, the air retention characteristics of the film is not acceptable as a bladder. Polyether-type TPU films do exhibit anti-hydrolysis properties. Although multiple pin-hole leaks are possible, the tear generally does not grow over 1/64 inch in length because the internal air pressure is greatly reduced once the leak begins and results in a reduction in stress on the films. Both water and stress are needed to propagate and continue the hydrolytic tear. An anti-hydrolysis urethane film was made by adding a dissimilar material that protects the urethane from the hydrolytic reaction. Adding an outer protective layer to the bladder construction greatly reduces the amount of water that can attack the polyester-type urethane inner layers. Materials that protect the inner urethane layer exhibit very low water transmission rates (less than 8 gram/mil/100 sq.in./24 hr). Specific materials with very low water transmission rates are polyvinylchloride, low density polyethylene, linear low density polyethylene, and ethylene-vinyl-acetate. These materials were also considered as protective films because of their flexibility (less than 10,000 psi flexural modulus) and stretchability (greater than 200% ultimate elongation).
Of the protective materials mentioned, only polyvinylchloride (PVC) was able to seal to the polyester-type TPU material in a multi-layer construction. Because the chemistries are similar and because they both are receptive to radio frequency sealing, the polyvinylchloride and polyester-type TPU combination was the best choice. Polyvinylchloride does not seal well, even to itself, using heat methods. The other low gas transmission rate films mentioned, all polyolefiln types, do not seal well using radio frequency method. Heat sealing is the transfer of heat from a source in order to melt the thermoplastic film layers, allow the polymers to flow, and cool to become one polymer in the sealed area. Radio-frequency sealing is the same procedure, only the molecular structure of the polymer is excited by radio waves that result in the polymer turning molten.
In testing, a polyester-type TPU bladder would leak, due to hydrolysis tearing, within 7 days when soaked in water for fifteen minutes and mechanically impacted 100,000 times. For a sport ball bladder constructed from a triple laminate of 5 mils of TPU, 7 mils of polyvinylchloride, and 5 mils of TPU, no failure due to leakage was witnessed after six months of observation. The test included the same water exposure and mechanical impacting as used in the TPU bladder test.
The outer layer of 5 mil polyester-type TPU film was found to provide an abrasion-resistant protective layer for the softer polyvinylchloride layer. The dimensions of 5-7-5 mils of the plies in this 3-ply film prove important in the processing and durability. Because the polyvinyl film exhibits lower tensile strength and ultimate elongation than the polyester-type TPU film, two additional mils were needed. The thicker polyvinylchloride film also resists cracking during impacting.
In the process of constructing a bladder, a sheet of material containing three total layers is formed. An injection molded valve of polyester-type TPU resin is placed in a die cut hole and sealed in place through heat- or radio-frequency methods. A second sheet formed from three layers of film is placed on top of the first sheet and all six layers of film are sealed at the periphery of the bladder by heat- or radio-frequency methods. Steel-rule-die blades cut through the layers of film on the outer edge of the seal to remove excess material.